Temperature control circuits

ABSTRACT

Temperature control circuits including a bridge network having an output controlling a programmable unijunction transistor, a load controlling the heat supplied to an area, an SCR in series with the load and controlled by the programmable unijunction transistor in accordance with temperature sensed by a thermistor in the bridge network, and a unidirectionally conductive limiter diode connected with the bridge network to permit control operation on only the initial portion of positive half cycles of a supply voltage. Modifications of the temperature control circuits include means for increasing temperature differentials to require discrete temperature changes to energize and deenergize the load.

United States Patent [72] Inventor Hugh J. Tyler Santa Ana, Calif.

[21] Appl. No. 878,336

[22] Filed Nov. 20, 1969 [45] Patented Feb. 16, 1971 [73] AssigneeRobertshaw Controls Company Richmond, Va.

[54] TEMPERATURE CONTROL CIRCUITS 3,385,957 5/1968 Munsonetal. 3,478,53211/1969 Cooteyetal Primary Examiner-Bernard A. Gilheany AssistantExaminer-F. E. Bell A ttorneys Auzville Jackson, J r.. Robert L. Marbenand Anthony A. O'Brien ABSTRACT: Temperature control circuits includinga bridge network having an output controlling a programmable 16 Claims 4Drawn; Figs unijunction transistor, a load controlling the heat suppliedto [52] U.S. Cl 219/499, an area, an SCR in series with the load andcontrolled by the 2 9/5 1 programmable unijunction transistor inaccordance with tem- [5l] l-l05b 1/02 perature sensed by a thermistor inthe bridge network, and a [50] 219/499, unidirectionally conductivelimiter diode connected with the 323/75 bridge network to permit controloperation on only the initial portion of positive half cycles of asupply voltage. Modifica- I References Cited tions of the temperaturecontrol circuits include means for in- UNITED STATES PATENTS creasingtemperature differentials to require discrete tempera- 3,149,224 9/1964Horne et al. 219/499 t r h ng to g z and deenergize the loadl4 f I8 N 24F. I6 I2 PATENT-ED FEB} 6 I97! sum 1 [1F 2 FIG. I

I INVENTOR Hugh J. Tyler ATTORNEY PATENTEU FEB 1' 6197! SHEET 2 OF 2FIG3 FIG; 4

INVENTOR Hugh J. Tyler ATTORNEY TEMPERATURE CONTROL CIRCUITS BACKGROUNDOF THE INVENTION rate detection of temperature changes as sensed bytemperature variable resistances. Another problem with prior arttemperature control circuits is that they permit rapid cycling of heatcontrolling devices such as switches for electric heating systems andvalves for fuel burning heating systems.

SUMMARY OF THEINVENTION The present invention is summarized in atemperature control circuit including a load adapted to control thetemperature in a space, a switch in series with the load and an ACsupply, a resistance bridge having a temperature variable resistance inone leg thereof, a defector for receiving output signals from the bridgecorresponding to the temperature in the space and controlling the.operation of the switch, and limiter means connectedacross another legof the resistance bridge, the limiter. means being conductive in onlyone direction and having a predetermined breakdown voltage whereby thelimiter means is triggered during the initial portion of positive halfcycles of the supply voltage to permit control of the switch independentofsupply voltage amplitude variations and the limitermeans isnonconductive during negative half cycles of the supply voltage.

An object of the present invention is to accurately control thetemperature in a space independent of supply voltage amplitudevariations.

Another object of the present invention is to increase the temperaturedifferential in a temperature control circuit to prevent rapid cyclingof a heating control device.

A further object of the present invention is to utilize aunidirectionally conducting limiter diode across one leg of a resistancebridge in a temperature control circuit.

Some of the advantages of the present invention over the prior art arethat a temperature'variable resistance is self heated over the entirecycle of supply voltage to increase tem perature differential, a singlediode is utilized to provide half wave rectification and operationindependent of supply voltage amplitude variations, and accurateoperation is provided without rapid cycling of a heating control,device.

Further objects and advantages of the present invention will becomeapparent from the following description of the preferred embodimentstaken in conjunction with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of atemperature control circuit embodying the present invention.

FIG. 2 is a set of curves for use in explaining the operation ofthecircuit of FIG. 1.

FIG. 3 is a schematic diagram of a modification of the circuitofFIG. I.

FIG. 4 is a schematic diagram of another modification of the circuit ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A temperature control circuitaccording to the Present invention is illustrated in FIG. 1 and includesa pair of supply terminals 10 and 12 adapted to receive a suitable ACsupply voltage. A load 14 is in series with a silicon controlledrectifier (SCR) 16 across input terminals 10 and 12. The junctionbetween load 14 and the anode of SCR l6 defines an input terminal 18 fora bridge network which further includes an input terminal 20 and outputterminals 22 and 24. Input terminal 20 is connected with the cathode ofSCR 16 such that the SCR is connected in shunt across the inputterminals of the bridge.

A pair of fixed resistors 26 and 28 are in series between inputterminals 18 and 20 of .thebridge with the junction therebetweendefining output terminal 24. A temperature sensing thermistor 30 is inseries with a variable resistance network 32 and a fixed resistor 34across input terminals :18 and 20 with the junction between resistivenetwork 32 and resistor 34 defining output terminal 22. Resistors 26, 28and 34 define resistive legs of the bridge and thermistor 30 andresistive network 32 define a fourth leg of the bridge. Resistivenetwork 32 includes a switch 36 which is adapted to be connected with aplurality of resistors 38, 40 and 42 all of which have a first terminaladapted to be connected with switch 36 and a second terminal connectedwith output terminal 22. Resistor 38 has an adjustable tap 44 shunting aportion of the resistor to output terminal 22. Resistor 38 has anadjustable tap 44 shunting a portion of the resistor to output terminal22. A unidirectionally conducting limiter=diode 46 is in parallel withresistor 34 with its anode connected to output terminal 22 and itscathode connected to input. terminal 20 and may be any diode having thecharacteristics of conducting in only one direction and only after apredetermined forward biasing voltage is placed thereacross, such as afour-layer or Schockley diode. I

Aprogrammable unijunction transistor (PUT) 48 has its anode connectedwith output terminal 22, its gate connected with output terminal 24 andits cathode connected with the gate of 'SCR 16. The operation ofprogrammable unijunction transistors is well known in the art; however,in brief, the operation of the PUT 48 is such that the programmableunijunction transistor is conductive only on positive half cycles of thesupply voltage and only when the anode voltage at output terminal 22 isgreater than the gate voltage at output terminal 24.

The load 14 may be any type of control device such as a relay or valve,the operation of which controls the heating in a defined space, such asan oven. Thermistor 30 is disposed in the oven to sense the temperaturetherein and has a positive temperature coefficient of resistance suchthat the temperature in the oven corresponds to the resistance exhibitedby thermistor 30. The switch 36 in resistive network 32 is moved tocontact any one of the resistors 38, 40 and 42 which have varyingresistances associated therewith defining various set temperatures forthe oven. For instance, resistor 38 may constitute a conventional ovencontrol such that tap 44 may be set by manipulation of a knob outside ofthe oven to set the desired oven operating temperature. Resistor 40 mayhave an extremely small resistance associated with automatic cleaning ofthe oven at very high oven temperatures and resistor 42 may correspondto a standard oven temperature for specific cooking methods, such asbroiling. r

In operation, the required set temperature for operation of the oven isdetermined by properly setting resistive network 32 such that thevoltage at output terminal 22 will be greater than the voltage at outputterminal 24 only when the temperature sensed by thermistor 30 is lessthan the desired operating temperature of the oven as set by resistivenetwork 32', The supply voltage received at terminals 10 and I2 issupplied to input terminals 18 and 20 of the bridge; and, sinceresistors 26 and 28 are fixed, the voltage at output terminal 24 will bea substantially constant percentage of the supply voltage. The voltageat output terminal 22 will vary in accordance with the resistance ofthermistor30; and, due to the voltage divider effect between thermistor30 and resistive network 32, and fixed resistor 34, the voltage atoutput terminal 22 will decrease as the temperature in the ovenincreases.

When the oven is initially turned on thetemperature therein will be lowand the voltage at output terminal 22 will be greater than the voltageat output terminal 24 due to the low 'tion is provided during thenegative half cycle.

E The second cycle illustrated in FIG.;2 represents a conditionwhere-theoven temperature hasdecreasedsuch'that the voltat outputterminal 24; however, PU-T 48 is not triggered when I the anode and gatevoltages are equal.

I The third cycle illustrated in FIG;

, "ductiile and deenergizingload 14.;

I When thetemperature in the ovenis below the set tempera minal 22'willfail toexceed the-voltage at'output terminal 24; and, accordingly, PUT48 willfbe' rendered nonconductive thereby rendering SCR l6nonconductive. The resistance of thebridge is of such a value thatload14 will not receive suffi- *cient electrical energy to'en ergize the.control device when SCR 16 is nonconducting; and, accordingly, thesupply of heat I to the oven will be terminated.

The'operation of the temperature control circuit will be a moreparticularly described with theaid of FIG. 2. The supply voltagereceived at terminals an'djl2 is indicated at 50. A

solid curve 52 is representative of the voltage at output terminal 24 ofthe bridge, and a dashed-curve 54 is representative of the voltage atoutput terminal 22 of the bridge.A curve 56 I represents the output onthe cathode of PUT 48, and the voltage across load 14 isindicated;at-58.

Assuming'thetemperature in the oven to be above the set temperatureduring the first cycle of'supply voltage-50,;it can beseen that thevoltage' 54 on loutputterminal 22 rises slower .than the voltage onoutput terminal 24;.and, consequently,

. PUT 48 will not'be triggered.,Thepredetermined voltage at ,whichlimiter diode 46breaksidown is set such that'the limiter diode 46willbreak down shortly aftercommencement of the diode 46 breaks down,PUT 48 cannotbe triggered and output terminal 22is essentially clampedtoterminal 12 shunting resistor 34. Since the anode voltage of PUT 48will not have ob- .tained at valuegreater than the gate voltage, PUT 48willnot be triggered during this cycle; and, accordingIy,.SCR 16 willremain nonconducting to permit only a small voltage to be ap- 7 pliedtoload 14, which voltage'is insufficient to place load 14 in an energizedstate. During-the negative half cycle of the supply voltage SCR l6,limiter diode 46, and PUT 48 block any reverse'current flowtherethrou'gh,'and thus no control acageat output terminal22 isapproximately equal to thevol'tage 2 represents acondition' wheretheoven temperature decreases below the set tempera ture such thatthevoltage atoutput-terminal 22 rises faster than the voltage at outputterminal 24 thereby triggering PUT 48't'o provide a trigger pulse 56 tothe gate of SCR 16.,When SCR 16 is rendered conductive the voltage dropacross load 14 is substantiallyincreased as shown by curve 58 such thattheload will be placed in its e nergized state to supply more v heat tothe oven. SCR 16 is reriderednonconducting on negathe temperaturedifferential of the temperature control circuit to provide more uniformcycling.

It may be seen that limiter diode 46 provides the function of qutilizingonly the initial portion' 'of the positive half-cycle of the supplyvoltage for detecting oven'temperature whereby the voltage detected isindependent of the amplitude of the supply voltage and adverse effectsfrom supply voltage variations are avoided, as well as the halfwave-rectifying function of rendering the temperature control systemoperable only on positive half cycles. Furthermore, the unidirectionalconducting characteristic of limiter diode 46 provides theabovedescribed increase in temperature differential to reduce cycling of loadA modification of the temperature control circuit of FIG. I

is illustrated in FIG. 3 and components in the temperature controlcircuit of FIG. 3 which are identical to components of FIG. 1 are givenidentical reference-numbers and arenot described again.

A resistor 60 is in series with limiter diode 46 across resistor .34,anda diode 62 has its anode connected to the junction of resistor 60 andthe cathode of .limit'er diode 46. The cathode of diode 62 is connectedto output terminal'24 through a resistor 64-and-to supply terminal 12through a capacitor 66.

l The operation of the modified temperature control circuit of FIG. 3 isthe same as that previously described with respect positive half cycleof supplyvoltage 50; and, once limiter to the embodiment of FIG. I;however, the additional circuitry in the modification of FIG. 3 preventsrapid cycling of the load by requiring ,a small but. definite resistancechange at thermistor 30 in order to deenergiz'e and energize load 14.

' During positive half cycles of the supply'voltage when-the oven.temperature is greater than the set temperature, limiter diode 46 willbe triggered to pass'a current through resistor 60 and develop a. smallpositive voltage thereacrossThesmaII positive voltage is utilized to'charge capacitor 66 through diode 62andthe voltageoncapacitor 66 isapplied tooutput terminal 24 of the bridge through resistor, 64.-'I'hus, during operation of the circuit when the oven temperature.isabove theset temperature, the voltage applied to the gate of PUT 48from output terminal 24 of the bridge is slightly higher than normalthereby requiring an increased voltageat output ter-' minal 22 totrigger PUT 48. Once the oven temperature [decreases sufficiently totriggerPUT-48, limiter diode 46 willbe rendered n'onconductive duringthe entire positive half cycles; and thus, the slightadditional voltageadded-to output terminal 24 disappears. Consequently, due to thereduction in voltage atoutput terminal 24.,the voltage required atoutput tive half cycles of supply voltage however, the holdingcharacteristic ofload I4 is such'thattheload will remain inits energizedstate when SCR 16 is energized on consecutive posi- I tive half cycles.Thus, the oven will continue to be heated until the oven temperature assensed by thermistor 30increases to the set temperature such that thePUT 48 will'not be triggered on' positive halt cycles therebymaintaining SCR I6 nonconture,xlimite r. diode 46 is triggered at thebeginning of each positive half cycle and continues to conduct for theremainder of the positive half cycle. The current through limiter diode46 also flows through thermistor 30 and'causes a small amount ofself-heating dependent upon the design of the thermistor. When the oventemperature exceeds the set temperature,

tively shuntsthe'bridge and reduces the amount of current flowingthrough thermistor 30;This' action tends to increase terminal 22 torender PUT48nonconductive is decreased by n a small butde'finite amountthereby requiring a definite in- -,-crease in oven temperaturepriortodeenergizationof load 14.

It can be seen, therefore, thatjthe circuit of FIG.-'3.prevents rapidcycling of Ioadl4.

Another modification of the temperature control circuit of FIG. 1 isillustrated in'FlG. 4.and components inthe temperature control circuitof FIG'. which are identical to components of FIG. 1 are given identicalrefercncenumbers and are not described again.

v Limiter diode 46 is connected to supply terminal IZ-through a resistor68, and the series combination of a diode 70nd a. capacitor 72 isconnected in parallel with resistor 68. The anode'of diode 70' isconnected with the cathodeof limiter diode 46 such that diodes 46 and 70are-similarly poled and the junction of capacitor 72 with the cathode ofdiode 7 .0-is

. PUT 48 is triggered to render-SCR 16 conductive which effec- Iconnected with the base of an NPN transistor 74 through a rev sistor'76.The emitter of transistor 74 is connected directly to supplyterminal 12and the collector of transistor 74 is .con-

nected with a .tap' for a variable'resistor 78. Fixed resistor 34isconnected to ground through a resistor 80,- andthe variable resistor 78and the emitter-collector path of transistor 74 shunt register 80.

t The operation of the same'as that previously described with respect tothe embodiment of FIG. I; however, thev embodiment of FIG. 4provides alarge adjustable temperature differential, thereby renderingmodificationof FIG. 4 is essentially the i the circuit useful for thoseapplications where the temperature control circuit is used to sense atemperature corresponding to a limit of safe operation. That is, oncethe load 14 is deenergized in response to temperature, it is desireableto permit the device being controlled, such as an oven, to coolconsiderably before commencing operating again.

When the temperature sensed by thermistor 30 is above the settemperature, a current is passed by limiter diode 46 after breakdown todevelop a voltage across resistor 68 which is utilized to chargecapacitor 72 through diode 70. The voltage across capacitor 72 applies apositive potential to the base of transistor 74 to render the transistorhighly conductive thereby shunting resistor 80 and effectively reducingthe voltage at output terminal 22. Once the temperature se'nsed'bythermistor 30 decreases sufficiently to trigger PUT 48, limiter diode 46will cease conducting to thereby render transistor 74 nonconducting andremove the shunt from across resistor 80 thereby immediately raising thevoltage at output terminal 22. Consequently, a sufficientincrease in theresistance of thermistor 30 is requiredbefore voltage at output terminal22 will decrease sufficiently to render PUT 48 nonconductive anddeenergize load 14. The temperature differential may be adjusted bymovement of the tap on resistor 78 to vary the resistance shunted acrossresistor 80.

The increased temperature differentials provided by the modificationsonFIGS. 3 and 4 are extremely reliable since the temperature differentialsare provided without the use of mechanical contacts.

if so desired, the embodiments of FIGS. 1, 3 and 4 may be altered suchthat limiter diode 46 is connected across fixed resistor 26 to providesimilar operation with the realization that the modified circuitry ofFIGS. 3 and 4 may be rearranged to operate with limiter diode '46 in itsnew position.

in as much as the present invention is subject to many variations,modifications and changes in detail, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitedsense. 8

I claim:

1. A temperature control circuit comprising:

supply means adapted to receive an AC supply voltage;

load means connected with said supply means and adapted to control thetemperature in a space;

switch means and controlling the energization of said load I means; I

a bridge network including input means connected with said supplyterminal means to receive the supply voltage, output means, threeresistive legsconnected with said input means and said output means, anda fourth leg having temperature variable resistance means disposed insaid space and connected with said input means and said output meanssuch that output signals are provided at said output means correspondingto the temperature in said space;

detection means connected with output means to receive said outputsignal and assume a conductive or nonconductive state in accordancetherewith, said detection means having an output connected with saidswitch means to control the operation thereof such that said means isconductive'when said detection means is conductive to energize said loadmeans; and

' limiter means connected with said input means and said output means ofsaid bridge network across one of said resistive legs, said limitermeans being unidirectionally conductive and having a predeterminedbreakdown voltage whereby said limiter means is triggered during theinitial portion of positive half cycles of the supply voltage to permitcontrol of said switch means independent of amplitude variations of thesupply voltage and said limiter means is nonconductive during negativehalf cycles of the supply voltage.

2. The invention as recited in claim 1 wherein. storage means isconnected with said limiter means to change the output signals at saidoutput means independent of said temperature variable resistance meansto increase the temperature differential of said temperature controlcircuit.

3. The invention as recited in claim 2 wherein said limitcr meansincludes a unidirectionally conducting breakdown diode, and said storagemeans includes a capacitor connected in series with said breakdowndiode.

4. The invention as recited in claim 3 wherein said capacitor isconnected with said output means to increase the voltage of the outputsignals at said output means.

5. The invention as recited in claim 3 wherein said storage meansincludes shunt switch means shunting a portion of said one resistive legand connected with said capacitor whereby said shunt switch means isconductive in response to voltage on said capacitor to decrease thevoltage of the output signals at said output means.

6. The invention as recited in claim 5 wherein saidjshunt switch meansis a transistor.

7. The invention as recited in claim 1 wherein said input means includesa first input terminal and a second input terminal, said output meansincludes a first output terminal and a second output terminal, said oneresistive leg is connected between said first input terminal and saidfirst output terminal. a second of said resistive legs is connectedbetween said first input terminal and said second output terminal, athird of said resistive legs is connected between said second inputterminal and said second output terminal, said fourth leg is connectedbetween said second input terminal and said first output terminal, andsaid detection means is connected with said second output terminal toreceive reference signals and with said first output terminal to receiveoutput signals corresponding to the temperature in said space.

8. The invention as recited in claim 7 wherein said limiter meansincludes a unidirectionally conducting breakdown diode.

9. The invention as recited in claim 8 wherein a capacitor is connectedin series with said breakdown diode through a second diode having ananode connected with the cathode of said breakdown diode and a cathodeconnected with a first side of said first input terminal.

10. The invention as recited in claim 9 wherein said cathode of saidbreakdown diode is connected with said first input terminal through afirst resistor, said first side of said capacitor is connected with saidsecond output terminal through a second resistor, said detection meansincludes a semiconductor switch having a first electrode connected withsaid first output terminal and a third electrode connected with saidsecond output terminal and a third electrode connected with said switchmeans, said semiconductor switch. beingconductive during positive halfcycles of the supply voltage when the voltage of the output signals atsaid first input terminal is greater than the voltage of the referencesignals at said second output terminal whereby said capacitor is chargedwith said semiconductive switch is non'conductive to add voltage to thereference signals at said second output terminal.

11. The invention as recited in claim 10 wherein said semiconductorswitch is a programmable unijunc'tion transistor having an anodeconnected with'said first output terminal, a gate connected with saidsecond input terminal and a cathode, and said switch'means is a siliconcontrolled rectifier having an anode connected with. said second outputterminal and said load means, a cathode connected with said first inputterminal, and a gate connected with said cathode of said programmableunijunction transistor.

12. The invention as recited in claim 11 wherein said fourth leg of saidbridge includes variable resistance means for setting the desiredtemperature to be maintained in said space.

13. The invention as recited in claim 9 wherein said cathode of saidbreakdown diode is connected with said first input terminal through afirst resistor, said first side of said first capacitor is connectedwith the base of a transistor, the emitter-collector path of-saidtransistor being connected across a portion of said one resistive leg ofsaid bridge and said detection means includes semiconductor switchhaving a first electrode connected with said first output terminal, asecond electrode connected with said second output terminal and a thirdelectrode connected with said switch means, said semiconductor switchbeing conductive during positive half cycles of the supply voltage whenthe voltage of .the' output signals at said first input terminal isgreater than thevoltage of the reference signals at said second outputterminal whereby said capacitor is charged when said semiconductorswitch is nonconductive to bias'said transistor into a conductive stateto shunt said portion of said one resistive leg to decrease the voltageof the output signals at said first input terminal.

14. The invention as recited in claim 13 wherein said emitter-collectorpath of said transistor is connected across said portion of said oneresistive leg through a second variable resistor and a third resistor isconnected with said first side of said capacitor and the base of saidtransistor.

15. The invention as recited in claim 14 wherein said semiconductorswitch is a programmable unijunction transistor having an anodeconnected with said first output terminal, a gate connected with saidsecond input terminal and a cathode, and said switchmeans is a siliconcontrolled rectifier having an anode connected with said second inputterminal and said load means. a cathode connected with said first inputterminal, and a gate connected with said cathode of said programmableunijunction transistor.

16. The invention as recited in claim 15 wherein said fourth leg of saidbridge includes variable resistance means for setting the desiredtemperature to be maintained in said space.

1. A temperature control circuit comprising: supply means adapted to receive an AC supply voltage; load means connected with said supply means and adapted to control the temperature in a space; switch means and controlling the energization of said load means; a bridge network including input means connected with said supply terminal means to receive the supply voltage, output means, three resistive legs connected with said input means and said output means, and a fourth leg having temperature variable resistance means disposed in said space and connected with said input means and said output means such that output signals are provided at said output means corresponding to the temperature in said space; detection means connected with output means to receive said output signal and assume a conductive or nonconductive state in accordance therewith, said detection means having an output connected with said switch means to control the operation thereof such that said means is conductive when said detection means is conductive to energize said load means; and limiter means connected with said input means and said output means of said bridge network across one of said resistive legs, said limiter means being unidirectionally conductive and having a predetermined breakdown voltage whereby said limiter means is triggered during the initial portion of positive half cycles of the supply voltage to permit control of said switch means independent of amplitude variations of the supply voltage and said limiter means is nonconductive during negative half cycles of the supply voltage.
 2. The invention as recited in claim 1 wherein storage means is connected with said limiter means to change the output signals at said output means independent of said temperature variable resistance means to increase the temperature differential of said temperature control circuit.
 3. The invention as recited in claim 2 wherein said limiter means includes a unidirectionally conducting breakdown diode, and said storage means includes a capacitor connected in series with said breakdown diode.
 4. The invention as recited in claim 3 wherein said capacitor is connected with said output means to increase the voltage of the output signals at said output means.
 5. The invention as recited in claim 3 wherein said storage means includes shunt switch means shunting a portion of said one resistive leg and connected with said capacitor whereby said shunt switch means is conductive in response to voltage on said capacitor to decrease the voltage of the output signals at said output means.
 6. The invention as recited in claim 5 wherein said shunt switch means is a transistor.
 7. The invention as recited in claim 1 wherein said input means includes a first input terminal and a second input terminal, said output means includes a first output terminal and a second output terminal, said one resistive leg is connected between said first input terminal and said first output terminal, a second of said resistive legs is connected between said first input terminal and said second output terminal, a third of said resistive legs is connected between said second input terminal and said second output terminal, said fourth leg is connected between said second input terminal and said first output terminal, and said detection means is connected with said second output terminal to receive reference signals and with said first output terminal to receive output signals corresponding to the temperature in said space.
 8. The invention as recited in claim 7 wherein said limiter means includes a unidirectionally conducting breakdown diode.
 9. The invention as recited in claim 8 wherein a capacitor is connected in series with said breakdown diode through a second diode having an anode connected with the cathode of said breakdown diode and a cathode connected with a first side of said first input Terminal.
 10. The invention as recited in claim 9 wherein said cathode of said breakdown diode is connected with said first input terminal through a first resistor, said first side of said capacitor is connected with said second output terminal through a second resistor, said detection means includes a semiconductor switch having a first electrode connected with said first output terminal and a third electrode connected with said second output terminal and a third electrode connected with said switch means, said semiconductor switch being conductive during positive half cycles of the supply voltage when the voltage of the output signals at said first input terminal is greater than the voltage of the reference signals at said second output terminal whereby said capacitor is charged with said semiconductive switch is nonconductive to add voltage to the reference signals at said second output terminal.
 11. The invention as recited in claim 10 wherein said semiconductor switch is a programmable unijunction transistor having an anode connected with said first output terminal, a gate connected with said second input terminal and a cathode, and said switch means is a silicon controlled rectifier having an anode connected with said second output terminal and said load means, a cathode connected with said first input terminal, and a gate connected with said cathode of said programmable unijunction transistor.
 12. The invention as recited in claim 11 wherein said fourth leg of said bridge includes variable resistance means for setting the desired temperature to be maintained in said space.
 13. The invention as recited in claim 9 wherein said cathode of said breakdown diode is connected with said first input terminal through a first resistor, said first side of said first capacitor is connected with the base of a transistor, the emitter-collector path of said transistor being connected across a portion of said one resistive leg of said bridge and said detection means includes semiconductor switch having a first electrode connected with said first output terminal, a second electrode connected with said second output terminal and a third electrode connected with said switch means, said semiconductor switch being conductive during positive half cycles of the supply voltage when the voltage of the output signals at said first input terminal is greater than the voltage of the reference signals at said second output terminal whereby said capacitor is charged when said semiconductor switch is nonconductive to bias said transistor into a conductive state to shunt said portion of said one resistive leg to decrease the voltage of the output signals at said first input terminal.
 14. The invention as recited in claim 13 wherein said emitter-collector path of said transistor is connected across said portion of said one resistive leg through a second variable resistor and a third resistor is connected with said first side of said capacitor and the base of said transistor.
 15. The invention as recited in claim 14 wherein said semiconductor switch is a programmable unijunction transistor having an anode connected with said first output terminal, a gate connected with said second input terminal and a cathode, and said switch means is a silicon controlled rectifier having an anode connected with said second input terminal and said load means, a cathode connected with said first input terminal, and a gate connected with said cathode of said programmable unijunction transistor.
 16. The invention as recited in claim 15 wherein said fourth leg of said bridge includes variable resistance means for setting the desired temperature to be maintained in said space. 